The present invention relates to a hydrocarbon sensor.
Hitherto, a hydrocarbon sensor using barium-cerium oxide as a solid electrolyte has been proposed (Japanese Laid-open Patent No. 9-127055).
A schematic structure of this hydrocarbon sensor is shown in FIG. 12. Reference numeral 101 is a solid electrolyte layer composed of barium-cerium oxide, and a cathode 102 and an anode 103 are formed on its surface by thick film printing process. On the solid electrolyte layer 101, a ceramic substrate 104 made of forsterrite is adhered and fixed with an adhesive of inorganic material (inorganic adhesive) 108. A heater 109 is formed on the surface of the ceramic substrate 104 by a thick film printing process. In part of the adhesion layer adhered by the inorganic adhesive 108, a diffusion rate-determining hole 111 (area indicated by dotted line in FIG. 12) for introducing hydrocarbon gas (HC) is provided.
The operation of this hydrocarbon sensor is explained. As shown in FIG. 12, a constant voltage is applied to the cathode 102 and anode 103. An ammeter for detecting the output is provided in the circuit. The solid electrolyte layer 101 is heated by a heater 109 in order to activate. In this state, when the HC passes through the diffusion rate-determining hole 111 to reach the anode 103, the HC is decomposed, and protons conduct through the solid electrolyte layer 101. As a result, current I flows in the circuit. The magnitude of this current I increases in proportion to the amount of protons, that is, the concentration of the HC. Therefore, the HC concentration can be detected from the output of the ammeter.
The hydrocarbon sensor having such structure is capable of obtaining a linear output corresponding to the HC concentration, and is free from effect of oxygen if coexisting while the HC concentration is low. In other words, the hydrocarbon sensor excellent in gas selectivity is obtained.
However, when applying such hydrocarbon sensor in HC detection in automobile emission, the solid electrolyte layer 101 must be activated. Accordingly, when the current flows in the heater 109, the ceramic substrate 104 on which the heater 109 is formed is subject to a significant stress, and known that the yield is lowered. In this mechanism, the following reasons are considered.
1) The coefficient of thermal expansion of the ceramic substrate 104 (forsterrite) on which the heater 109 is formed is 11 to 11.5xc3x9710xe2x88x926/xc2x0 C., whereas the coefficient of thermal expansion of the solid electrolyte layer 101 is about 10xc3x9710xe2x88x926/xc2x0 C., and the difference of the two is more than 1xc3x9710xe2x88x926/xc2x0 C.
2) The HC in the emission is a reducing atmosphere, and the ceramic substrate 104 is exposed to it, and part of composition (MgO forsterrite) of the ceramic substrate 104 is reduced, and the strength is reduced.
3) The platinum paste of the thick film for forming the heater 109 is a porous and uneven composition, and the current flow is concentrated in this area, and the temperature becomes high locally.
4) The heater 109 is formed at one side of the ceramic substrate 104 only, and this formed side is an exposed structure, and when the current flows in the heater 109 is this state, a temperature difference occurs suddenly between the heater forming side and the back side (the adhesion side with the solid electrolyte layer), and a large stress occurs.
The conventional hydrocarbon sensor also has other problem, that is, when the hydrocarbon sensor is installed in the automobile emission, the emission temperature varies with the engine running state, and the temperature of the hydrocarbon sensor may vary as much as 620xc2x130xc2x0 C. That is, the temperature variation width is 60xc2x0 C. The output current of the hydrocarbon sensor varies not only with the HC concentration, but also with the temperature, and such temperature variation may cause to lower the accuracy of hydrocarbon sensor. This is because the temperature regulation precision of the heater 109 is not sufficient.
The invention solves these problems, and it is hence an object thereof to present a hydrocarbon sensor excellent in yield and high in detection precision.
To solve the problems, the hydrocarbon sensor of the invention comprises a solid electrolyte layer composed of barium-cerium oxide, a pair of electrodes provided on the solid electrolyte layer, a ceramic substrate having a coefficient of thermal expansion nearly same as that of the solid electrolyte layer, and a heater provided on the ceramic substrate, in which the solid electrolyte layer and ceramic substrate are bonded to each other.
Further, at the heater forming side of the ceramic substrate, an auxiliary substrate having a coefficient of thermal expansion nearly same as that of the ceramic substrate is provided.
Further, the heater comprises control means for controlling on/off switching of the heater, comparing means for comparing the resistance value of the heater and the target resistance value of the heater being predetermined corresponding to the temperature, and judging means for suppressing the output from the control means depending on the signal from the comparing means.
In this constitution, the hydrocarbon sensor excellent in yield and high in detection precision is obtained.